A hologram is that in which two lights having the same wavelength (object light and reference light) are allowed to interfere with each other to record the wave front of the objective light as an interference fringe on a photosensitive material. When the hologram is irradiated by a light having the same condition as that of the original reference light, a diffraction phenomenon due to the interference fringe occurs, and the same wave front as that of the original object light can be reproduced.
The hologram is classified into various kinds according to the recording form of the interference fringe. A so-called volume hologram in which the interference fringe is recorded according to the difference of refractive index in the inside of a recording layer has recently been used for applications such as three-dimensional display, optical element, etc. because of its high efficiency and excellent wavelength selectivity.
As the photosensitive material for recording the volume hologram, for example, silver halide or gelatin dichromate have hitherto been used in the field of art. However, since these materials require wet development or complicated fixing treatment, it is not suitable to produce the hologram industrially. Further, there is a disadvantage that the recorded image may disappear due to moisture.
In order to solve the above disadvantage, U.S. Pat. Nos. 3,658,526 and 3,993,485 suggest the preparation of a volume hologram using a photopolymer by a simple dry treatment. Further, a mechanism of forming a hologram using a photopolymer is estimated in Applied Optics, B.L. Booth, Vol. 14, No. 3, pages 593-601 (1975); Applied Optics, W. J. Tomlinson & E. A. Chandross, Vol. 15, No. 2, pages 534-541 (1976) and the like. However, these techniques were inferior to the above prior art in refractive index modulation which is one of the important properties. U.S. Pat. Nos. 4,942,102 and 4,942,112 suggest the improvement of it by using a non-reactive plasticizer. However, the hologram thus formed is inferior in film strength and the refractive index modulation is not sufficiently improved.